1. Field of the Invention
The present invention relates to a signal transmission structure, and more particularly to a signal transmission structure adapted to reduce impedance mismatch on signal lines resulting from the plating bar and to enhance the resonant frequency thereof.
2. Description of the Related Art
In circuit substrates, signal lines applied thereon to couple two ports or components and used for transmitting signals, especially high frequency or speed, signals should be kept at a constant width for maintaining characteristic impedance thereof. Moreover, the impedance mismatch of the signal lines for transmitting the signals should be improved to reduce the reflection of signals stemming therefrom. Therefore, to reduce the insertion loss of the signal transmission and to enhance the return loss of the signals can improve the transmission quality thereof.
In a prior art circuit substrate, such as a chip carrier for chip package, after the formations of the transmission circuit and the solder mask layer, an anti-oxidation layer, such as a Ni/Au layer, is electroplated on bonding pads of the transmission circuit for avoiding the oxidation of Cu therein. Therefore, the adhesion of the bonding pads is enhanced. For electroplating the anti-oxidation layer, the bonding pads of the transmission circuit are coupled to an external power via a plating bar.
As to wire bonding technology, die bonding, wire bonding, molding and trimming processes are sequentially performed after the formation of the anti-oxidation layer. During the trimming process, the plating bar is trimmed thereby for forming a plurality of chip packages. It should be noted that the plating bar cannot be completely removed during the trimming and resides thereon.
FIG. 1 is a schematic stereo configuration of a prior art signal transmission structure. The signal transmission structure 100 is adapted for a circuit substrate, such as a chip carrier. The signal transmission structure 100 comprises a signal line 110, a plating bar 120, a ground plane 130 and a power plane 140. The ground plane 130 is over the power plane 140. The signal line 110 is composed of a first signal line 110a, a via 110b and a second signal line 110c. The first signal line 110a is disposed on the ground plane 130, and the second signal line 110c is disposed under the power plane 140. The first signal line 110a, the ground plane 130, the power plane 140 and the second signal line 110c can be sequentially formed on each layer of the four-layer patterned circuit substrate. In addition, the via 110b, through the ground plane 130 and the power plane 140, electrically connects the first signal line 110a and the second signal line 110b. 
Referring to FIG. 1, the plating bar 120 and the first signal line 110a are on the ground surface 130. One end of the plating bar 120 connects with the first signal line 110a which results in an induced capacitance between the plating bar 120 and the ground plane 130. Therefore, the characteristic impedance at the conjunction of the signal line 110 and the plating bar 120 varies. Because of the impedance mismatch of the signal line 110 resulting from the plating bar 120, a big insertion loss or a small return loss occurs which results in the incompletion of the transmission of signals from one end to the other of the signal line 110.
To resolve the issue of impedance mismatch, the prior art reduces the width of the plating bar for enhancing the impedance thereof. Although the issue can be resolved by the approach, it still cannot efficiently reduce the effective permittivity of the plating bar and enhance the resonant frequency thereof for the high frequency or speed signal transmission structures. Therefore, under this circumstance, the transmission quality of high frequency signals cannot be substantially improved.